1. Field of the Invention
This invention relates to an optical glass having a high refractive index and a low optical dispersion.
2. Description of the Prior Art
Most high refractive and low dispersive optical glass compositions hitherto known generally contain thorium oxide as a component to make them highly refractive and less dispersive. However, thorium is a radioactive element which is injurious to humans.
Optical glass having a high refractive index and a low optical dispersion, and which is free of thorium is known; for example, a B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Y.sub.2 O.sub.3 ternary system. Such system, however, has a high liquifaction temperature, and therefore, has the disadvantage of devitrification. The tendency to devitrification does not allow any appreciable increase in the amount of La.sub.2 O.sub.3 and Y.sub.2 O.sub.3. As a result, it has been impossible to attain a stable supply of optical glasses having a high refractive index, and which are free of dangerous thorium.